Chapter 22 Descent with Modification A Darwinian View of Life Chapter 22 1 The Darwinian Revolution Challenged Traditional Views of a young Earth inhabited by unchanging species Darwin proposed that life s diversity arose from ancestral species through natural selection a departure from prevailing views In contrast to catastrophism the principle that events in the past occurred suddenly by mechanisms not operating today Hutton and Lyell thought that geologic change results from mechanisms that operated in the past in the same manner as at the present uniformitarianism Lamarck hypothesized that species evolve but the underlying mechanisms he proposed are not supported by evidence Chapter 22 2 Descent with modification by natural selection explains the adaptions of organisms and the unity and diversity of life Darwin s Research Darwin became a clergyman and later joined the HMS Beagle as a conversation partner to captain Robert Fitz Roy The Voyage of the Beagle Darwin left with the Beagle in 1831 Darwin collected thousands of South American plants and animals He was inspired by Lyell s Principles of Geology The beagle stopped on the volcanic islands of the Galapagos there he noticed the differences in several types of species i e finches Darwin s Focus on Adaptation Adaptations inherited characteristics of organisms that enhance their survival and reproduction in specific environments Darwin hypothesized that a new species can arise from an ancestral for by the gradual accumulation of adaptations to a different environment Natural selection a process in which individuals that have certain inherited traits tend to survive and reproduce at higher rates than other individuals because of those traits In 1858 Alfred Wallace published a manuscript describing the same evolutionary ideals as Darwin Darwin eventually published On the Origin of Species by Means of Natural Selection in his book he explained the 3 broad observations about nature o The unity of life o The diversity of life o The match between organisms and their environments Descent with Modification descent with modification summarized Darwin s view of life Organisms share many characteristics leading Darwin to perceive unity in life o He attributed the unity of life to the descent of all organisms from an ancestor that lived in the remote past Darwin views the history of life as a tree with multiple branching from a common trunk to the tips of twigs representing the diversity of organisms Artificial Selection Natural Selection Adaptation Artificial selection the selective breeding of domesticated plants and animals to encourage the occurrence of desirable traits Observation 1 members of a population often vary in their inherited traits o Inference 1 individuals whose inherited traits give them a higher probability of surviving and reproducing in a given environment tend to leave more offspring than other individuals Observation 2 all species can produce more offspring than their environment can support and many of these offspring fail to survive and reproduce o Inference 2 this unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations An organisms heritable traits can influence not only its own performance but also how well its offspring cope with environmental challenges Darwin reasoned that natural selection should be capable of substantial modification of a species over many hundreds of generations With time natural selection will increase the frequency of individuals with favorable adaptations and hence refine the match between organisms and their environment Main ideas of natural selection o Natural selection is a process in which individuals that have certain heritable traits survive and reproduce at a higher rate than other individuals because of those traits o Over time natural selection can increase the match between organisms and their environment o If an environment changes or if individuals move to a new environment natural selection may result in adaptation to these new conditions sometimes giving rise to new species Individuals do not evolve it is the population that evolves with time Chapter 22 3 Evolution is supported by an Overwhelming Amount of Scientific Evidence Four types of Evidence o Direct observation of evolutionary change o Homology o Fossil records o Biogeography Direct Observation of Evolutionary Change An example is the soapberry bug whose beak changed in response to their food source In some areas the food sources are different and the bugs beak reflects the difference In example soapberry bugs in south Florida have a hollow needle like mouth compared to central Floridian soapberry bugs Another example of ongoing natural selection is the evolution of drug resistant pathogens Such as S aureus which becomes resistant to penicillin within a few years Natural selection is a process of editing not a creative mechanism Natural selection depends on time and place it favors those characteristics in a genetically variable population that provide advantage in the current local environment What is beneficial in one situation may be harmful in another Homology Homology similarity resulting from common ancestry Homologous structures structures in different species that are similar because of common ancestry Vestigial structures remnants of features that served important functions in the organism s ancestors o In example the skeletons of some snakes retain vestiges of the pelvis and leg bones of walking ancestors Evolutionary tree a diagram that reflects evolutionary relationships among groups of organisms Convergent evolution the independent evolution of similar features in different lineages o in example the sugar glider vs the flying squirrel Analogous these features share similar function but not common ancestry o while homologous features share common ancestry but not necessarily similar function The Fossil Record Fossils can shed light on the origins of new groups of organisms Fossils show the evolutionary changes that have occurred in various groups of organisms The fossil record shows that over time descent with modification produced increasingly large differences among related groups of organisms ultimately resulting in the diversity of life that we see today Biogeography Biogeography the geographic distribution of species o Can be influenced by continental drift the slow movement of land masses from Pangaea Endemic
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